The present invention relates to a YIG (yttrium iron garnet) thin film microwave apparatus including means for applying a D.C. bias magnetic field to a microwave device using ferrimagnetic resonance of a YIG thin film.
There has been proposed a microwave apparatus such as a filter and an oscillator which utilizes ferrimagnetic resonance of a YIG thin film as a ferrimagnetic material formed on a GGG (gadrinium gallium garnet) non-magnetic substrate by liquid phase epitaxial growth (which will be hereinafter referred to as LPE) and worked in a desired shape such as a circular or rectangular shape by selective etching with a photolithography technique. The microwave apparatus makes it possible to form a microwave integrated circuit with a transmission line such as a micro-strip line, and easily effect hybrid connection with another microwave integrated circuit. Further, since the microwave apparatus utilizing magnetic resonance of the YIG thin film may be prepared by the LPE and the photolithography technique as mentioned above, mass productivity is improved.
As mentioned above, the microwave apparatus utilizing magnetic resonance of the YIG thin film has practical advantages over a conventional magnetic resonance device using a YIG sphere.
However, in the microwave apparatus utilizing ferrimagnetic resonance of the YIG thin film as aforementioned, ferrimagnetic resonance frequency f of the YIG thin film is largely dependent upon temperature T. Therefore, there arises a significant problem in practical use that temperature characteristics are not satisfactory.
Such a problem will be described below. The ferrimagnetic resonance frequency f of the YIG thin film may be expressed in the following manner by using a Kittel's equation, provided that an anisotropy field contribution to the resonance frequency is small enough to be neglected. EQU f(T)=.gamma.{Hg(T)-Nz.sup.Y 4.pi.Ms.sup.Y (T)} (1)
Where, .gamma. is a gyromagnetic ratio, .gamma.=2.8 MHz/Oe; Hg is a D.C. bias magnetic field; Nz.sup.Y is a demagnetization factor of the YIG thin film, which factor is calculated by using a magnetostatic mode theory; and 4.pi.Ms.sup.Y is a saturation magnetization of the YIG. All of f, Hg and 4.pi.Ms.sup.Y is a function of temperature T. In one example of perpendicular resonance of a YIG disk having an aspect ratio (thickness/diameter) of 0.01, the demagnetization factor Nz.sup.Y is 0.9774, and the saturation magnetization 4.pi.Ms.sup.Y is 1916 G (Gauss) at -20.degree. C., and 1622 G at +60.degree. C., supposing that the bias magnetic field Hg is constant irrespective of temperature. Accordingly, the resonance frequency f is varied 835 MHz in the temperature range of -20.degree. C. to +60.degree. C.
In order to avoid deviation in the resonance frequency due to environmental temperature in the YIG thin film microwave apparatus, there has been proposed a method for maintaining the YIG thin film magnetic resonance device at a constant temperature by locating the device in a thermostatic chamber, or a method for maintaining the resonance frequency of the device constant by changing a magnetic field in dependence on temperature by means of an electromagnet. However, such a method as above necessitates an external energy supply means such as a current controller to make the constitution complicated.